Social Bookmarking
"On a social bookmarking system or network, users store lists of Internet resources that they find useful. These lists can be accessible to the public by users of a specific network or website. Other users with similar interests can view the links by topic, category, tags, or even randomly." (Source: Wikipedia)

Social Bookmarking relies on tags, or user-defined keywords, to organize content. Tagging is the practice of adding tags (keywords) to content such as photos, blog articles, video clips and bookmarks to organize, categorize and make them searchable. Tagging is closely associated with the concept of folksonomies, or user-defined taxonomies.

Delicious - http://del.icio.us/
A social bookmarking service that allows you to store, organize and share your bookmarks online. Add tags (keywords), annotations and bundles. Subscribe to others' bookmarks or to specific tags to create a personal resource network.

Furl - http://www.furl.net/
"By bookmarking any site you find of interest on Furl, you can easily annotate, find it later, refer to it, and share it with others. All on a system with practically unlimited storage, accessible to you from anywhere on the web, at no cost to you."

Zeitgeist Bookmarking

Digg - http://digg.com/
"Digg is democratizing digital media. As a user, you participate in determining all site content by discovering, selecting, sharing, and discussing the news, videos, and podcasts that appeal to you."

Technorati - http://technorati.com/
" Technorati is the recognized authority on what's happening on the World Live Web, right now. The Live Web is the dynamic and always-updating portion of the Web. We search, surface, and organize blogs and the other forms of independent, user-generated content (photos, videos, voting, etc.) increasingly referred to as 'citizen media'."